1. Field
Example embodiments of inventive concepts relate to a lamp cover for light emitting device.
2. Description of the Related Art
Combining the uses of blue LEDs and blue excited phosphor is a popular technology to produce white light emission. In such a technology, the blue light hits the phosphor and is partially converted into a longer wavelength, such as a yellow one. The excited light portion mixed with the residual blue light produces a pseudo white light.
A common method to implement such a white light emitting device is to mix phosphor with a liquid resin binder and dispense the phosphor mixed resin binder around the LED chip. The main disadvantage of this method is that it is hard to get a uniform color distribution at different angles since the light path length at different angles cannot be easily controlled to be the same. Meantime, the displacement of LED dies during die bonding process make the uniform color control more difficult.
To get a more uniform color distribution, a method to use a pre-made phosphor structure with the same thickness at all position is proposed in U.S. Pat. No. 7,582,914. In such a method, the pre-made phosphor structure is placed far away from the LED chip. In many cases, it is separated from the blue LED package and attached onto the PCB, where the blue LED package is located. Since the phosphor structure is far away from the LED chip and the leadframe house cavity where the LED chip is located, the scattering effect of the phosphor particles is much less than that in the phosphor-near-chip case so that blue light can easily escape without being absorbed to emit fluorescent light. As a result, phosphor usage and weight percentage should be greatly increased to make the phosphor structure for achieving the same correlated color temperature of light emission in comparison to the phosphor-near-chip case. The bigger the distance between the phosphor structure and the LED chip is, the more phosphor usage is needed, which dramatically increases the cost of lumen per watt for white LED lighting system since the current commercial phosphor prices are very expensive.
As for general lighting, a pre-made phosphor structure with a big dimension or large area is highly desired since it not only provides a uniform light distribution, but also reduces the LED flickering issue, which has been complained by customers for long time.